The present invention relates to an apparatus and method for detecting trace amounts of nitrocompounds. It provides a novel means for in situ detection of the presence of NO.sub.x containing compounds, wherein x is greater or equal to one, in the atmosphere or on various surfaces. The invention has been developed in order to monitor the presence of energetic materials, such as propellants and explosives, and nitropollutants in various environments.
Detection and monitoring of trace amounts of energetic materials is critical to the security and safety of the population. It is of particular interest in the areas of detecting potential terrorist activity, demilitarization and mine countermeasures. As a result of interest in this area, numerous methods and devices have been developed to reduce the risk to the general population by detecting the presence of said materials and preventing their use. The techniques which have been previously developed and employed are summarized in an article by A. Fainberg, entitled "Explosives Detection for Aviation Security," Science, Vol. 255, p. 2531 (March 1992).
The techniques previously employed include x-ray detection and nuclear techniques, such as thermal neutron analysis and nuclear resonance absorption. These techniques have been found to be applicable in the sensing of concealed energetic materials found in luggage or other containers. However, it is extremely difficult to detect the presence of dangerous chemicals--i.e. explosives--when carried by a person. Due to the health risks in exposing a human to x-rays and nuclear techniques, authorities are unable to monitor people in the same fashion or as thoroughly as they can luggage or other containers.
The above described prior art techniques are used to physically detect the presence of the target compounds. However, other prior art methods do exist which serve the purpose of detecting the presence of the target compounds'vapors in the atmosphere. Among the prior art vapor sensing techniques employed are gas chromatography/chemiluminescence, quadrupole mass spectrometry, ion mobility spectrometry, and animals, such as sniffing dogs.
Although these techniques appear attractive since they are capable of atmospheric vapor analysis, their applications are limited. For instance, when one employs the sensing method of gas chromatography/chemiluminescence, the response time before the results are obtained is lengthy, while quadrupole mass spectrometry is limited in that it suffers from nonselective ionization, that is all the species entering the ionization region are ionized and transmitted into the mass spectrometer. An additional technique previously employed is ion mobility spectrometry. This method is extremely sensitive (parts per trillion (ppt) range) and has a relatively short response time (seconds). However, this method is not quantitatively as accurate as the others since the signal dependence on concentration is non-linear, particularly in the tens of parts-per-billion (ppb) range. Moreover, with ion mobility spectrometry, clustering of the target molecules with water, competition for protonation with contaminants, and low mass spectroscopic resolution are problems frequently encountered.
The final prior art method to be discussed for detecting the presence of the specific materials of interest in the atmospheric vapor is the use of canines. The dog is the original vapor detector, or sniffer. Authorities still employ dogs to sweep buildings or sniff luggage or parcels to assure the absence or detect the presence of explosives therein. The use of dogs for these purposes is limited due to the fact that they are subject to fatigue, behavioral variations (good days/bad days), and the need for reinforcement and retraining. Although canines are very sensitive to the presence of specific vapors, they are subject to various outside factors. Therefore, it is critical to develop an efficient technological method for detecting specific vapors wherein said method is not effected by the detrimental factors set forth above. It is these factors which reduce the effectiveness of the method employed.
The present invention makes use of laser technology for the detection of trace atmospheric nitro compounds. Although lasers are not commonly employed in the detection of energetic materials of the type discussed above, they have been employed in the remote and in situ detection of toxic and non-toxic pollutant gases. In particular, a mobile light detection and ranging system (LIDAR) has been employed to detect NO and NO.sub.2 in the ppb ranges at distances of less than 10 kilometers. This absorption method detects NO and NO.sub.2 only, and not the energetic nitrocompounds which contain these functional groups. It employs the technique of differential absorption and is used commonly for remote detection. Many large molecules including energetic materials are difficult to detect spectroscopically using this technique due to the lack of distinguishing structure or absence of any absorption features in the ultraviolet-visible (UV-VIS) spectral region.
Although this laser technique has been used in the remote and in-situ detection of pollutant gases, the LIDAR methods are based on laser absorption which is a fundamentally different measurement as well as a less efficient means of detection than that employed herein. The present invention is for point detection and utilizes a laser differently than in the LIDAR technique. The invention herein employs a laser to both generate local levels of NO/NO.sub.2 fragments from nitrocompounds and for the ionization is the NO fluorescence and/or ionization of NO that is being monitored to determine the presence of energetic nitrocompounds. This is accomplished using a photodetector and/or ion detector.
Accordingly, the present invention satisfies the recognized need for the development of a technological device capable of detecting trace amounts of atmospheric and surface-adsorbed nitrocompounds of the types discussed above, wherein said device and method employing the device are both accurate and efficient. The present invention is designed to effectively reduce the risk factor described--i.e., the presence of explosives, mines, etc.